This invention relates to a process of preparing crystalline calcium hydrogen phosphate anhydride suitable as a raw material for phosphate phosphors.
Calcium hydrogen phosphate anhydride CaHPO.sub.4 is an important material for phosphate phosphors used in fluorescent lamps such as calcium halophosphate phosphors represented by the general formula Ca.sub.5 (PO.sub.4).sub.3 (Cl,F):Sb,Mn. Such phosphors need to be produced in high purity and with strict control of particle shape, particle size and size distribution since these factors affect fluoresecent characteristics of the produced phosphors and workability of the phosphors in phosphor coating operations during production of fluorescent lamps. Accordingly calcium hydrogen phosphate anhydride as the raw material is required to be almost free of impurities such as SO.sub.4, Na, Fe, Cl, etc. Furthermore, various standards have been established by the manufactures of phosphate phosphors for crystal shape, particle size, size distribution and Ca/P mole ratio of the anhydrous phosphate.
Calcium hydrogen phosphate anhydride is obtained by the dehydration of calcium hydrogen phosphate dihydrate CaHPO.sub.4.2H.sub.2 O. In a conventional process, the phosphate dihydrate to be converted to the anhydride for use in the production of phosphate phosphors is prepared by a reaction of a refined and, hence, costly phosphoric acid with a refined calcium salt at a temperature near room temperature. The precipitated phosphate dihydrate is dispersed in water either together with the mother liquor or after separation from the mother liquor, and the resultant slurry is heated to 70.degree.-100.degree. C. to cause the conversion of the phosphate dihydrate to the anhydride. Alternatively, the precipitated phosphate dihydrate is separated from the mother liquor and heat-dried at 100.degree. C. or above until completion of the conversion of the dihydrate to the anhydride. In another conventional process, calcium hydrogen phosphate dihydrate is precipitated by reaction of diammonium phosphate with calcium chloride, and the conversion of the precipitated phosphate dihydrate to the phosphate anhydride is accomplished by maintaining a slurry of the dihydrate in a heated state for a sufficiently long period of time with precise control of the temperature. However, calcium hydrogen phosphate anhydride prepared by any of these conventional processes is not fully satisfactory in its purity, crystal shape and particle size as a material for phosphors. Besides, these conventional processes are rather unfavorable from an economical viewpoint.
U.S. Pat. No. 4,044,105 shows a process of preparing high purity calcium hydrogen phosphate anhydride in the form of platy and square-shaped crystals of a desirable particle size (as a material for phosphors). In this process, the phosphate dihydrate is prepared by reaction of sodium ammonium hydrogenphosphate NaNH.sub.4 HPO.sub.4.4H.sub.2 O with calcium chloride in an aqueous phase and is dispersed in water to give a 5-20% slurry. After adjustment of the pH of the slurry to 4.5-5.5, the slurry is rapidly heated to 85.degree.-97.degree. C. by contact with steam to cause conversion of the phosphate dihydrate to the anhydride. However, in some cases the product of this process may be still unsatisfactory in its Na or Fe content, and it is impossible to obtain lozenge-shaped crystals of the phosphate anhydride with a Ca/P mole ratio below 1.01 by this process despite an existing demand for the phosphate anhydride of such a quality.